The Oxford Dictionary for Writers and Editors is an invaluable and relatively inexpensive tool which is invaluable for checking the spelling of awkward words, as well as other issues of language usage such as spelling and capitalization.

Grammars

Practical English UsageMichael Swan
Oxford University Press, 2005 (3rd edition), ISBN-10: 019442099
An alphabetical guide to the most common problems of English grammar.

Oxford Guide to English GrammarJohn Eastwood
Oxford University Press, 1994, ISBN-10: 0194313514
A basic English grammar, organized according to parts of speech.

Fowler's Modern English Usage
R.W. Burchfield
Oxford University Press, 2004 (3rd edition), ISBN-10: 0198610211
A more detailed alphabetical guide to English grammar.